In a pyroelectric material in which a spontaneous polarization has been formed by being polarized in advance, the surface charges caused by the spontaneous polarization are neutralized by floating charges of the atmosphere at normal times. However, infrared rays are irradiated to the pyroelectric material, and the temperature of the pyroelectric material is varied as much as AT. In accordance with the temperature variation, the size of the spontaneous polarization are varied. However, the floating charges cannot respond to the temperature variation as fast as the spontaneous polarization, and therefore, the variation amount appears as the surface charges.
Thereafter, the initial neutral state is restored. If this variation of the surface charges is converted into signals, then a pyroelectric infrared ray sensor is formed.
The pyroelectric material is a signal source of a high impedance of 10.sup.9 -10.sup.12 .OMEGA., therefore, if signals are to be extracted from the high impedance signal source, an impedance matching has to be carried out. Therefore, if a pyroelectric material of a large impedance is to be used as an infrared ray sensor, an impedance matching is carried out by connecting a field effect transistor as shown in FIG. 1, so that signals can be extracted from the pyroelectric material (which is the high impedance (10.sup.9 -10.sup.12 ) signal source).
Thus the pyroelectric sensor is capable of sensing an object in a non-contacting manner by receiving infrared rays from the object and based on the variation of the surface charges.
This pyroelectric sensor can produce a pyroelectric output only when the object shows a variation in the infrared ray radiation. Therefore, it can detect only a temperature variation of an object or a moving object. In the case of an object other than a temperature varying body or as moving body, the incident infrared rays are chopped by means of a chopper so as to irradiate chopped infrared rays which are chopped into angular frequencies .omega.. In this way, voltage outputs which correspond to the unit infrared ray sensitivity can be obtained.
FIG. 1 illustrates the constitution of a conventional pyroelectric infrared ray sensor.
Referring to this drawing, a rear side 13 of a pyroelectric sensor 10 is provided with two reflecting electrodes 11 and 12. Another side 13' of the pyroelectric sensor 10 is provided with two optical receiving electrodes 11 and 12 which correspond to the reflecting electrodes 11 and 12, so that pyroelectric output can be obtained from the reflecting electrodes 11 and 12.
Further, two output terminals of the pyroelectric sensor 10 is connected through a wire 14 to a high resistance 21 which is provided on a substrate 20. The voltage drop between the pyroelectric sensor 10 and the high resistance 21 is compensated by carrying out an amplification by means of a field effect transistor 23.
An equivalent circuit for such a pyroelectric infrared ray sensor is illustrated in FIG. 2.
However, the above described conventional technique has the following disadvantages. That is, a high resistance 21 having a value of 10.sup.12 .OMEGA. is required, and a wire 14 for connecting the reflecting electrodes 11 and 12 of the pyroelectric sensor 10 to the high resistance 21 is required.
Particularly, the conventional sensor requires a high resistance, and therefore, the enlargement of the bulk cannot be avoided.
In an attempt to overcome the above described disadvantages, a study was made to use a thin film resistor. However, ruthenium oxide (RuO) was to be used for the thin film resistor, or a polysilicon having no impurity diffusion was to be used for the thin film resistor. In the former, it is impossible to miniaturize to make integral with the impedance matching circuit, while in the latter, the variation of the resistance is severe between 10 .sup.9 and 10.sup.12 .OMEGA., although it is suitable to miniaturize.
In an attempt to overcome this problems, Japanese Patent Laid-open No. Sho-63-32328 discloses a device constituted as follows. That is, an impedance matching circuit is formed upon a silicon substrate, and an input resistor is made to contain about 1% of Cr, thereby forming a thin film resistor having a resistance variation range of 10.sup.8 -10.sup.13 .OMEGA..
However, in this pyroelectric sensor, an impedance matching circuit is formed upon a silicon substrate, and a wire bonding is carried out between the pyroelectric element and the input resistor. Therefore, a high precision technique is required in disposing the wire bonding between the semiconductor elements and the small devices.